Switching power supply device

ABSTRACT

A switching power supply includes: a first series circuit, connected to both terminals of a direct current power supply Vdc 1,  in which a primary winding  5   a  of a transformer T, a reactor L 3  and a first switch Q 1  are connected in series; a second series circuit, connected to both terminals of the primary winding  5   a  and the reactor L 3,  which includes a switch Q 2  and a capacitor C 3;  a smoothing circuit D 1,  D 2,  L 1,  C 4;  and a control circuit 10 alternately turning on and turning off the switches Q 1,  Q 2.  The transformer T includes: a main core  21,  formed with a magnetic circuit, on which the primary and secondary windings  5   a,    5   b  are wound with a given gap  23;  and a plurality of auxiliary cores  24   a,    24   b  disposed in the given gap  23  with a given distance in a circumferential direction of the primary winding  5   a.  Further, the reactor L 3  is formed of a leakage inductance of the transformer T.

TECHNICAL FIELD

The present invention relates to a switching power supply with highefficiency, small size, and low noises.

BACKGROUND ART

A switching power supply turns on or turns off a switch to control acurrent flowing through a primary winding of a transformer, andrectifies and smoothes a voltage developed across a secondary winding ofthe transformer, to supply the resulting current output to a load. Sincethe transformer used in such a switching power supply serves to transferenergy, a structure and a characteristic of the transformer playimportant roles.

FIG. 1 is a view showing a structure of one example of a magneticleakage transformer disclosed in Japanese Patent Application Laid-openNo. 2000-340441. The magnetic leakage transformer 111 shown in FIG. 1 iscomprised of an E-core 113, formed with a magnetic circuit, which ismade of magnetic material, an I-core 115 that forms a main iron coretogether with the E-core 113, a primary winding 119 and a secondarywinding 125 mounted to the E-core 113 at suitable locations, acylindrical magnetic leakage iron core 123, made of magnetic material,which is located in a position through which magnetic fluxes leaked fromthe magnetic circuit pass, and a current detection winding 121 mountedonto the magnetic leakage iron core 123 for detecting leakage magneticfluxes.

In the magnetic leakage transformer 111, since the magnetic leakage ironcore 123 made of magnetic material is located in a position throughwhich magnetic fluxes leaked from the magnetic circuit pass and thecurrent detection winding 121 is mounted onto the magnetic leakage ironcore 123 for detecting the leakage magnetic fluxes, the current can bedetected without causing power loss and complexity in structure.

DISCLOSURE OF THE INVENTION

In the meanwhile, research and development work has been progressivelyundertaken in recent days to provide a switching power supply wherein areactor is connected to a primary winding of a transformer in series toallow a switch to execute switching operations through the use of energystored in the reactor for thereby minimizing switching losses. In such astructure, in order to further eliminate the switching losses, thereactor needs to have a well-suited inductance value.

However, since the magnetic leakage transformer 111 merely has a fixedleakage inductance, the inductance of the reactor have beenappropriately adjusted by connecting an external reactor to the primarywinding 119 in series. This results in an increase in the number ofcomponent parts, an increase in costs, an increase in a packagingsurface area and an increase in size of the switching power supply.

It is, therefore, an object of the present invention to provide aswitching power supply wherein a transformer has a well-suited leakageinductance value with no external reactor while making it possible toachieve zero-voltage switching for thereby providing high efficiency anda minimized structure at low noises.

The present invention has been completed with a view to addressing theabove issues. A first aspect of the present invention provides aswitching power supply comprising a first series circuit, connected toboth terminals of a direct current power supply, in which a primarywinding of a transformer, a reactor and a first switch are connected inseries, a second series circuit, connected to both terminals of thefirst switch or both terminals of the primary winding and the reactor,in which a second switch and a capacitor are connected in series, asmoothing circuit smoothing a voltage developed across a secondarywinding of the transformer, and a control circuit alternately turning onand turning off the first and second switches, wherein the transformerincludes a main core, formed with a magnetic circuit, on which theprimary and secondary windings are wound with a given gap, and aplurality of auxiliary cores disposed in the given gap with a givendistance in a circumferential direction of the primary winding, andwherein the reactor is formed of a leakage inductance of thetransformer.

A second aspect of the present invention provides a switching powersupply comprising a first series circuit, connected to both terminals ofa direct current power supply, in which a primary winding of atransformer, a reactor and a first switch are connected in series, asecond series circuit, connected to both terminals of the first switchor both terminals of the primary winding and the reactor, in which asecond switch and a capacitor are connected in series, a smoothingcircuit smoothing a voltage developed across a secondary winding of thetransformer, a control circuit alternately turning on and turning offthe first and second switches, and a feedback winding located on asecondary side of the transformer to allow energy stored in the reactor,when the first switch is turned on, to be circulated to a secondary sidewhen the first switch is turned off, wherein the transformer, formedwith a magnetic circuit, including: a main core that has a central legon which the primary winding of the transformer and the feedback windingare wound with a given gap, and a side core on which the secondarywinding of the transformer is wound, and a plurality of auxiliary coresdisposed in the given gap with a given distance in a circumferentialdirection of the primary winding, and wherein the reactor is formed of aleakage inductance of the transformer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view illustrating a structure of one example of a relatedart magnetic leakage transformer.

FIG. 2 is a circuit structural view showing a switching power supply ofa first embodiment.

FIG. 3 is a timing chart for signals at various parts of the switchingpower supply of the first embodiment.

FIG. 4 is a timing chart illustrating details of signals appearing atvarious parts of the switching power supply of the first embodiment whena switch Q1 is turned on.

FIG. 5 is a view illustrating a B-H characteristic of a transformerincorporated in the switching power supply of the first embodiment

FIG. 6 is a timing chart of a current flowing through a saturablereactor incorporated in the switching power supply of the firstembodiment

FIG. 7 is a structural view showing a concrete example 1 of an innerbobbin of the transformer.

FIG. 8 is a structural view showing a concrete example 1 of an outerbobbin of the transformer.

FIG. 9 is a structural view showing a concrete example 2 of an innerbobbin of the transformer.

FIG. 10 is a structural view showing a concrete example 2 of an outerbobbin of the transformer.

FIG. 11 is a circuit structural view showing a switching power supply ofa second embodiment.

FIG. 12 is a structural view showing a transformer incorporated in theswitching power supply of the second embodiment

FIG. 13 is a timing chart for signals at various parts of the switchingpower supply of the second embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, switching power supplies of various embodiments accordingto the present invention are described below in detail with reference tothe accompanying drawings.

Embodiment 1

A switching power supply of a first embodiment features that when a mainswitch is turned on, electric power is directly supplied to a load via asecondary winding of a transformer while when the main switch is turnedoff, excitation energy stored in a primary winding of the transformer isstored in a capacitor C3 to turn on an auxiliary switch whereby firstand third quadrants on a B-H curve of a core of the transformer is usedand making up shortfalls in the excitation energy with energy from areactor L3 allows a starting point on the B-H curve to rest on a lowerend of the third quadrant whereas a saturable reactor is connected tothe primary winding of the transformer in parallel whereby the saturablereactor is saturated at a terminating point of an on-period of theauxiliary switch to increase a current to cause a reverse voltage tosharply occur when the auxiliary switch is turned off for therebypermitting the main switch to achieve zero-voltage switching.

FIGS. 2A, 2B and 2C are circuit structural views of the switching powersupply of the first embodiment. In FIG. 2A, connected across bothterminals of a direct current power supply Vdc1 is a series circuit thatis composed of a reactor L3, a primary winding 5 a (with the wingingnumber n1) and a switch Q1 (main switch) such as a MOSFET. Connectedacross both terminals of the switch Q1 are a diode D3 and a resonantcapacitor C1 in parallel.

The diode D3 may include a parasitic capacitor of the switch Q1 and theresonant capacitor C1 may include a parasitic capacitor of the switchQ1.

Connected to a junction between one end of the primary winding 5 a ofthe transformer T and one end of the switch Q1 is one end of a switch Q2(auxiliary switch) composed of a MOSFET. Further, other end of theswitch Q2 is connected to a positive electrode of the direct currentpower supply Vdc1 and one end of the reactor L3 via a capacitor C3.Also, the other end of the switch Q2 may be connected to a negativeelectrode of the direct current power supply Vdc1 via the capacitor C3.

The reactor L3 forms an electric power supply source that storeselectric power when the switch Q1 is turned on and supplies storedelectric power to the capacitor C3 when the switch Q1 is turned off.

Connected across the both terminals of the switch Q2 in parallel theretois a diode D4. The diode D4 may include a parasitic diode of the switchQ2. The switches Q1, Q2 have a duration (dead time period) in which bothswitches Q1, Q2 are turned off and are alternately turned on or turnedoff by PWM controls performed by a control circuit 10.

Connected across the both terminals of the primary winding 5 a of thetransformer T is a saturable reactor SL1. The saturable reactor SL1utilizes a saturating characteristic of a core of the transformer T.Since a large alternating current with equal amplitude flows through thesaturable reactor SL1, magnetic fluxes equally vary between first andsecond quadrants in terms of a zero point on a B-H curve shown in FIG.5.

As shown in FIG. 5, further, magnetic fluxes B (more properly, thesymbol B represents a magnetic density and a true magnetic fluxes φ aredescribed as φ=B·S where S represents a cross-section area of a core. Inthis case, S=1 and φ=B are supposed) saturate at Bm in terms of acertain positive magnetic field H and saturate at -Bm in terms of acertain negative magnetic field H. The magnetic field H is generated inproportion to the magnitude of a current i. With the saturable reactorSL1, the magnetic fluxes B shift in a path: Ba→Bb→Bc→Bd→Be→Bf→Bg on theB-H curve and the magnetic fluxes have a wide operating range. Statesbetween Ba and -Bb, Bf and Bg on the B-H curve represent saturatedstates, respectively.

Wound on a core of the transformer T are the primary winding 5 a and asecondary winding 5 b (with the winding number n2) having the same phaseto the primary winding 5 a. Further, one end of the secondary winding 5b is connected to a diode D1 while a junction between the diode D1 andone end of a reactor L1 and the other end of the secondary winding 5 bare connected to a diode D2. Furthermore, the diodes D1 and D2 form arectifying circuit. The other end of the reactor L1 and the other end ofthe secondary winding 5 b are connected to a capacitor C4. The capacitorC4 smoothes a voltage of the reactor L1 to output a direct currentoutput to a load LR.

Also, the transformer T forms a structure as shown in a fontcross-sectional view in FIG. 2B and in a side cross-sectional view inFIG. 2C, whose detail is described below.

The control circuit 10 alternately and controllably turns on or turnsoff the switches Q1 and Q2 and controls in a way to decrease anon-duration of a pulse applied to the switch Q1 while increasing anon-duration of a pulse applied to the switch Q2 when an output voltageof the load RL exceeds a reference voltage. That is, a decrease of theon-duration of the pulse of the switch Q1 when the output voltage of theload RL exceeds the reference voltage allows the output voltage to becontrolled at a fixed voltage.

Further, the control circuit 10 turns off the switch Q2 at time when acurrent Q2 i of the switch Q2 increases and, thereafter, turns on theswitch Q1. When turning on the switch Q1, the control circuit 10 turnson the switch Q1 during a given period wherein the voltage of the switchQ1 becomes zero voltage due to a resonance between the resonancecapacitor C1 connected to the switch Q1 in parallel thereto and asaturated inductance of the saturable reactor SL1.

Next, a basic sequence of operations of the switching power supply ofthe first embodiment mentioned above is described with reference totiming charts shown in FIGS. 3, 4 and 6. FIG. 3 is the timing chart forsignals at various parts of the switching power supply of the firstembodiment. FIG. 4 is the timing chart illustrating details of signalsat various parts of the switching power supply of the first embodimentwhen the switch Q1 is turned on. FIG. 5 is a view showing a B-Hcharacteristic of a transformer incorporated in the switching powersupply of the first embodiment. FIG. 6 is the timing chart of a currentof a saturable reactor incorporated in the switching power supply of thefirst embodiment.

Also, FIGS. 3 ad 4 show a voltage Q1 v developed at the both terminalsof the switch Q1, a current Q1 i flowing through the switch Q1, avoltage Q2 v developed at the both terminals of the switch Q2, a currentQ2 i flowing through the switch Q2, and a current SL1 i flowing throughthe saturable reactor SL1.

First, if the switch Q1 is turned on at time t1 (corresponding to timet11 to t12), a current flows in a path: Vdc1→L3→5 a(SL1)→Q1→Vdc1. Then,energy is stored in the reactor L3. Also, at this time, a voltage isdeveloped across the second winding 5 b of the transformer T to cause acurrent to flow in a path: 5 b→D1→L1→C4→5 b. Moreover, a current SL1 ialso flows through the saturable reactor SL1 when the switch Q1 isturned on, and then energy is stored in the saturable reactor SL1.

As shown in FIG. 6, the current SL1 i varies on a current value a(negative value) at time t1, a current value b (negative value) at timet1 b, a current value c (zero current) at time t13, and a current valued (positive value) at time t2, in turn. On the B-H characteristic curveshown in FIG. 5, the magnetic fluxes vary in a sequence: Ba→Bb→Bc→Bd.Also, the symbols Ba to Bg shown in FIG. 5 correspond to the symbols ato g in FIG. 6.

Next, if the switch Q1 is turned off at time t2, energy stored in thesaturable reactor SL1 is charged to the capacitor C1. Then, a voltageresonance occurs between an inductance of the saturable reactor SL1 andthe capacitor C1, thereby raising the voltage Q1 v of the switch Q1.Also, a current flows in a path: L1→C4→D2→L1, thereby supplying acurrent to the load RL via the capacitor C4.

Then, when a voltage of the capacitor C1 equals that of the capacitorC3, the saturable reactor SL1 releases energy to render the diode D4conductive to allow the flow of diode current, thereby charging thecapacitor C3. Moreover, the switch Q2 is turned on and then the switchQ2 falls in a zero-voltage switch. Also, the current SL1 i varies from acurrent value d (positive value) at time t2 to a current e (zero) attime t20. On the B-H characteristic curve shown in FIG. 5, the magneticfluxes vary from Bd to Be.

Further, while the saturable reactor SL1 releases energy, energy isreleased from the reactor L3 in a path: L3→5 a(SL1)→D4→C3→L3, therebycharging the capacitor C3. That is, energy from the reactor L3 is addedto energy from the saturable reactor SL1 in the capacitor C3. Then, ifthe saturable reactor SL1 and the reactor L3 terminate releasingrespective energies, the charging of the capacitor C3 is stopped.

Next, during a period between time t20 and time t3, energy stored in thecapacitor C3 flows in a path: C3→Q2→SL1(5 a)→C3, thereby resetting themagnetic fluxes of the saturable reactor SL1. Likewise, the magneticfluxes vary in the transformer T to which the saturable reactor SL1 isconnected in parallel.

In this moment, since during the period between time t20 and time t3energy stored in the capacitor C3 is fed back to the saturable SL1, thecurrent SL1 i flowing through the saturable reactor SL1 i takes anegative value as shown in FIG. 6. That is, the current SL1 i variesfrom the current value e (zero) at time 20 to a current value f(negative value) at time t2 a. On the B-H characteristic curve shown inFIG. 5, the magnetic fluxes vary from Be to Bf. Also, a surface area Sdefined between time t2 and time t20 equals a surface area S definedbetween time t20 and time t2 a. This surface area S corresponds toenergy released from the saturable reactor SL1 and stored in thecapacitor C3.

Then, the current SL1 i varies from the current value f (negative value)at time t2 a to a current value g (negative value) at time t3. On theB-H characteristic curve shown in FIG. 5, the magnetic fluxes vary fromBf to Bg. A surface area defined between time t2 a and time t3corresponds to energy released from the reactor L3 and stored in thecapacitor C3.

That is, since energy stored in the capacitor C3 corresponds to an addedvalue of energy of the saturable reactor SL1 and energy of the reactorL3, the current SL1 i increases by a component equivalent to energysupplied from the reactor L3 during the resetting of the magneticfluxes. Thus, the magnetic fluxes shift to the third quadrant and reachto a saturable area (Bf-Bg) whereby the current SL1 i increases to markthe maximum level at time t3 (the same at time t1). The current SL1 iincreases immediately before the on-period of the switch Q2 terminatesand indicates a current appearing when the saturable reactor SL1 issaturated.

Further, at time t3, the current Q2 i of the switch Q2 is maximized.Turning off the switch Q2 at this time allows the capacitor C1 torapidly discharge and the current shortly becomes zeroed. Then, theswitch Q1 is able to achieve zero-voltage switching.

In such a way, since the switch Q1 performs the zero-voltage switchingdue to energy stored in the reactor L3 connected to the primary winding5 a of the transformer T, the zero-voltage switching is not completelyperformed in the presence of less energy in the reactor L3. Also, ifenergy is excessive, a circulating current increases, resulting in anincrease in loss of the switch Q1. For this reason, the reactor L3 needto have an appropriate inductance value.

(Basic Example of Transformer)

Therefore, with the first embodiment, it is featured that the reactor L3is connected to the primary winding 5 a of the transformer T in seriesin a structure wherein a leakage inductance between the primary winding5 a and the secondary winding 5 b of the transformer T is well-suited toallow the resulting leakage inductance to form the reactor L3 withoutrequiring an external reactor for thereby achieving the simplificationin a circuitry. That is, as shown in FIGS. 2B and 2C, auxiliary cores 24a and 24 b are inserted between the primary winding 5 a and thesecondary binding 5 b of the transformer T and adjusting the number ofauxiliary cores and a length L enables a leakage inductance value to beregulated for providing a desired reactor (inductor).

Describing a structure of the transformer T in detail, in FIGS. 2B and2C the primary winding 5 a and the secondary winding 5 b are mounted ona central leg 22 of a main core 21 having a B-shaped made of magneticmaterial, with a given spacing 23. Disposed in the given spacing 23 areauxiliary cores 24 a, 24 b, spaced from one another in a given distancealong a circumferential direction of the primary winding 5 a, which aremade of two magnetic materials with a given length L. Also, the primarywinding 5 a and the secondary winding 5 b have windings wound withinsulation tapes, respectively, though not shown.

Although this example has a structure that is composed of-two auxiliarycores, adjusting the number of auxiliary cores and the given length Lenables a well-suited leakage inductance value to be obtained. Further,increasing the number of auxiliary cores increases surface areas andincreasing the given length L shortens a gap between the main core 21and the auxiliary cores, resulting in an increase in a leakageinductance. Accordingly, appropriate energy is stored in the leakageinductance and this energy is able to allow the switch Q1 to completelyoperate in a zero-voltage switching mode.

Concrete Example 1 of Transformer T

FIG. 7 is a structural view showing a concrete example 1 of an innerbobbin of the transformer. FIG. 8 is a structural view showing aconcrete example 1 of an outer bobbin of the transformer. Thetransformer of the concrete example 1 is comprised of the main core 21shown in FIG. 2B, a cylindrical inner bobbin 31 (shown in FIG. 7) onwhich the primary winding 5 a is wound, and an outer bobbin 33 (shown inFIG. 8), formed with slits 34 a, 34 b each with a given length L along acircumferential direction of the primary winding 5 a to which auxiliarycores 35 a, 35 b are inserted, which is larger in diameter than thefirst bobbin 31 and on which the secondary winding 5 b is wound. Theinner bobbin 31 and the outer bobbin 33 are made of resin material.

By adjusting the number of auxiliary cores and the length L, awell-suited leakage inductance value is obtained. Further, for thepurpose of precluding the winding from dropping-off, the inner bobbin 31has both ends formed with stepped portions 31 a and, likewise, the outerbobbin 33 has both ends formed with stepped portions 33 a.

The transformer T with such a structure is fabricated in a mannerdescribed below. First, the slits 34 a, 34 b are formed in the outerbobbin 33 for mounting the auxiliary cores 35 a, 35 b and the secondarywinding 5 b is wound on the outer bobbin 33 on which the auxiliary cores35 a, 35 b each adjusted in the given length L are inserted to the slits34 a, 34 b.

Then, the primary winding 5 a is wound on the inner bobbin 31 and theinner bobbin 21 is inserted to the outer bobbin 33, on which the maincore 21 is mounted with the inner bobbin 31 inserted to the outer bobbin33, thereby completing the transformer T.

Thus, since the transformer T as fabricated above has the reactor L3with an appropriate inductance value and the insulation between theprimary winding 5 a and the secondary winding 5 b is performed with thebobbin, the transformer T has excellent insulation and less straycapacitance. Also, an electric power supply is enabled to have anincreased safety with a reduction in noises. Besides, no insulationtapes are needed for the primary winding 5 a and the secondary winding 5b, providing an ease of fabrication.

Concrete Example 2 of Transformer

FIG. 9 is a structural view showing a concrete example 2 of an innerbobbin of the transformer. FIG. 10 is a structural view showing aconcrete example 2 of an outer bobbin of the transformer. Thetransformer of the concrete example 2 is comprised of the main core 21shown in FIG. 2B, the cylindrical inner bobbin 31 (shown in FIG. 9) onwhich the primary winding Sa is wound, and an outer bobbin 37 (shown inFIG. 10), larger in diameter than the first bobbin 31 and wound with thesecondary winding 5 b, which is made of insulating magnetic materialsuch as plastic magnet material. The magnetic material may includeferrite and Permalloy.

By adjusting a magnetic permeability of insulation magnetic material, awell-suited leakage inductance value is obtained. Further, for thepurpose of precluding the winding from dropping-off, the inner bobbin 31has both ends formed with the stepped portions 31 a and, likewise, theouter bobbin 37 has both ends formed with stepped portions 37 a.

The transformer T with such a concrete example 2 has no use of theauxiliary cores, thereby enabling a transformer in a further simplifiedform.

Embodiment 2

Next, a switching power supply of a second embodiment is describedbelow. The switching power supply of the second embodiment features thata feedback winding is located on a secondary side (output side) of atransformer to increase an inductance value of a reactor connected to aprimary winding of a transformer and to cause energy, stored in thereactor when the switch Q1 is turned on, to be fed back to the secondary(output side) side.

FIG. 11 is a circuit structural view of the switching power supply ofthe second embodiment. Since the switching power supply of the secondembodiment, shown in FIG. 11, differs from the switching power supply ofthe first embodiment, shown in FIGS. 2A, 2B and 2C, in respect of atransformer Tb and associated peripheral circuitries of the transformerTb, only related parts are described.

In this example, the primary winding 5 a (with the winding number n1),the secondary winding (with the winding number n2), and a feedbackwinding 5 c (with the winding number n3) are wound on the transformerTb. The primary winding 5 a and the secondary winding 5 b are wound inthe same phase and the primary winding 5 a and the feedback winding 5 care wound in an opposite phase. That is, the secondary winding 5 b ofthe transformer Tb is loosely coupled to the secondary winding 5 a tocause a leakage inductance between the primary winding 5 a and thesecondary winding 5 b to form a reactor L3 connected to the transformerTb in series. Thus, energy, stored in the reactor L3 when the switch Q1is turned on, is fed back to the secondary side when the switch Q1 isturned.

One end (on a side ●) of the secondary winding 5 b and one end (on aside ●) of the feedback winding 5 c are connected at a junction to whichan anode of the diode D1 is connected. The other end (on a side absentof ●) of the feedback winding 5 c is connected to an anode of the diodeD2, and the cathodes of the diode D1 and the diode D2 are connected toone end of the capacitor C4. The other end of the capacitor C4 isconnected to the other end of the secondary winding 5 b (on a sideabsent of ●).

Next, operations of the switching power supply of the second embodimentwith such a structure are described with reference to a timing chartshown in FIG. 13. Also, FIG. 13 shows a voltage Q1 v developed at theboth terminals of the switch Q1, a current Q1 i flowing through theswitch Q1, a current Q2 i flowing through the switch Q2, a current SL1 iflowing through the saturable reactor SL1, and currents D1 i, D2 iflowing through the diodes D1, D2.

First, if the switch Q1 is turned on at time t1, a current flows in apath: Vdc1→L3→5 a(SL1)→Q1→Vdc1. Further, at this time, a voltage isdeveloped across the second winding 5 b of the transformer Tb to cause acurrent to flow in a path: 5 b→D1→C4→5 b. For this reason, as shown inFIG. 13, a current D1 i of the diode D1 linearly increases during aperiod from time t1 to time t2.

Next, if the switch Q1 is turned off at time t2, energy stored in thereactor L3 is circulated to the secondary side of the transformer Tb.That is, since a voltage is induced in the feedback winding 5 c on thesecondary side of the transformer Tb, a current flows in a path: 5c→D2→C4→5 b→5 c. Therefore, as shown in FIG. 13, a current D2 i flowsthrough the diode D2 during a period between times t2 to t3.

Thus, with the switching power supply of the second embodiment, aninductance value of the reactor L3 connected to the primary winding 5 aof the transformer Tb in series is set to an increased value, andenergy, stored in the reactor L3 when the switch Q1 is turned on, iscirculated to the secondary side of the transformer, providing animproved efficiency. Also, due to the diode D1 and the diode D2, asecondary current continuously flows during periods when the switch Q1is turned on or turned off. Thus, a ripple current of the capacitor C4decreases.

FIG. 12 is a structural view showing the transformer for use in theswitching power supply of the second embodiment. The transformer Tbshown in FIG. 12 includes a main core 41, formed in a B-shapeconfiguration, which has a central leg 42 on which the primary winding 5a and the feedback winding 5 c are mounted with a given gap 23. Disposedin the given gap 23 are auxiliary cores 24 a, 24 b, made of two magneticmaterials each with a given length L, which are spaced from each otherwith a given distance along a circumferential direction of the primarywinding 5 a. Also, the primary winding 5 a and the feedback winding 5 chave respective windings on which insulation tapes are wound,respectively, through not shown.

In this example, there are two auxiliary cores. However, adjusting thenumber of auxiliary cores and the given length L allows a well-suitedleakage inductance value to be obtained between the primary winding 5 aand the feedback winding 5 c.

Further, formed in the main core 41 are a pass core 43 a and a gap 44,and the secondary winding 5 b is wound on a side core 43. That is, theprimary winding 5 a is loosely coupled to the secondary winding 5 b bythe pass core 43 a, resulting in an increase in a leakage inductance.

Thus, devising a shape of the core of the transformer Tb and thewindings allows the primary winding 5 a, the secondary winding 5 b, andthe feedback winding 5 c to be coupled to a single main core 41 andproviding the pass core 43 a enables an increased leakage inductance tobe obtained to form the reactor L3. Thus, coupling the transformersection to the reactor enables the power switching supply to beminiaturized at low costs.

Further, locating a plurality of auxiliary cores in the gap 23 betweenthe primary winding 5 a and the feedback winding 5 c and adjusting thenumber of auxiliary cores and the given length L enables a leakageinductance to be adjusted to a well-suited value. Accordingly, theswitching power supply of the second embodiment has the same effects asthose of the switching power supply of the first embodiment.

Also, while the switching power supply of the second embodiment forms astructure in which the auxiliary cores 24 a, 24 b are located in thegaps 23 between the primary winding 5 a and the feedback winding 5 c,using the inner bobbin 31 shown in FIG. 7 and the outer bobbin shown inFIG. 8 may allow the primary winding 5 a on the inner bobbin 31 to bemounted on the inner bobbin 31 and the feedback winding 5 c to be woundon the outer bobbin 37 such that the inner bobbin 31 is mounted to thecentral leg 42 of the main core 41 under a condition where the innerbobbin 31 is inserted to the outer bobbin 37.

Further, using the inner bobbin 31 shown in FIG. 9 and the outer bobbin37 shown in FIG. 10 may allow the primary winding 5 a to be wound on theinner bobbin 31 and the feedback winding 5 c to be wound on the outerbobbin 37 such that the inner bobbin 31 is mounted to the central leg 42of the main core 41 under conditions where the inner bobbin 31 isinserted to the outer bobbin 37.

INDUSTRIAL APPLICABILITY

According to the present invention, by setting a leakage inductance ofthe transformer to a well-suited value, no external reactor is neededand zero-voltage switching is made operative, thereby enabling toprovide a switching power supply with high efficiency, low size, and lownoises.

Therefore, the switching power supply of the present invention isapplicable to a DC-DC conversion type power supply circuit and an AC-DCtype conversion type power supply circuit.

1. A switching power supply comprising: a first series circuit,connected to both terminals of a direct current power supply, in which aprimary winding of a transformer, a reactor, and a first switch areconnected in series; a second series circuit, connected to one of bothterminals of the first switch and both terminals of the primary windingand the reactor, in which a second switch and a capacitor are connectedin series; a smoothing circuit smoothing a voltage developed across asecondary winding of the transformer; and a control circuit alternatelyturning on and turning off the first and second switches, wherein thetransformer includes a main core, formed with a magnetic circuit, onwhich the primary and secondary windings are wound with a given gap, anda plurality of auxiliary cores disposed in the given gap with a givendistance in a circumferential direction of the primary winding, andwherein the reactor is formed of a leakage inductance of thetransformer.
 2. The switching power supply according to claim 1, whereinthe transformer includes: a cylindrical inner bobbin on which theprimary winding is wound; and an outer bobbin having a diameter largerthan that of the inner bobbin on which the secondary winding is wound,and having a plurality of slits, formed in a given distance along thecircumferential direction, which accommodate the plurality of auxiliarycores, respectively, and wherein the inner bobbin is mounted to the maincore under a condition where the inner bobbin is inserted to the outerbobbin.
 3. The switching power supply according to claim 1, wherein thetransformer includes: a cylindrical inner bobbin on which the primarywinding is wound; and an outer bobbin having a diameter larger than thatof the inner bobbin on which the secondary winding is wound, which ismade of insulating magnetic material, and wherein the inner bobbin ismounted to the main core under a condition where the inner bobbin isinserted to the outer bobbin.
 4. A switching power supply comprising: afirst series circuit, connected to both terminals of a direct currentpower supply, in which a primary winding of a transformer, a reactor,and a first switch are connected in series; a second series circuit,connected to one of both terminals of the first switch and bothterminals of the primary winding and the reactor, in which a secondswitch and a capacitor are connected in series; a smoothing circuitsmoothing a voltage developed across a secondary winding of thetransformer; a control circuit alternately turning on and turning offthe first and second switches; and a feedback winding, located on asecondary side of the transformer, which allows energy stored in thereactor when the first switch is turned on to be circulated to thesecondary side when the first switch is turned off, wherein thetransformer, formed with a magnetic circuit, including: a main core thathas a central leg on which the primary winding of the transformer andthe feedback winding are wound with a given gap and a side core on whichthe secondary winding of the transformer is wound; and a plurality ofauxiliary cores disposed in the given gap with a given distance in acircumferential direction of the primary winding, and wherein thereactor is formed of a leakage inductance of the transformer.
 5. Theswitching power supply according to claim 4, wherein the transformerincludes; a cylindrical inner bobbin on which the primary winding iswound; and an outer bobbin having a diameter larger than that of theinner bobbin on which the feedback winding is wound, and having aplurality of slits, formed in a given distance in the circumferentialdirection, which accommodate the plurality of auxiliary cores,respectively, and wherein the inner bobbin is mounted to the central legof the main core under a condition where the inner bobbin is inserted tothe outer bobbin.
 6. The switching power supply according to claim 4,wherein the transformer includes; a cylindrical inner bobbin on whichthe primary winding is wound; and an outer bobbin having a diameterlarger than that of the inner bobbin on which the feedback winding iswound, which is made of insulating magnetic material, and wherein theinner bobbin is mounted to the main leg of the main core under acondition where the inner bobbin is inserted to the outer bobbin.
 7. Theswitching power supply according to claim 1, further comprising: asaturable reactor connected to both terminals of the primary winding ofthe transformer to utilize a saturable characteristic of the core of thetransformer, wherein the control circuit turns off the second switchwhen a current of the second switch increases.
 8. The switching powersupply according to claim 2, further comprising: a saturable reactorconnected to both terminals of the primary winding of the transformer toutilize a saturable characteristic of the core of the transformer,wherein the control circuit turns off the second switch when a currentof the second switch increases.
 9. The switching power supply accordingto claim 3, further comprising: a saturable reactor connected to bothterminals of the primary winding of the transformer to utilize asaturable characteristic of the core of the transformer, wherein thecontrol circuit turns off the second switch when a current of the secondswitch increases.
 10. The switching power supply according to claim 4,further comprising: a saturable reactor connected to both terminals ofthe primary winding of the transformer to utilize a saturablecharacteristic of the core of the transformer, wherein the controlcircuit turns off the second switch when a current of the second switchincreases.
 11. The switching power supply according to claim 5, furthercomprising: a saturable reactor connected to both terminals of theprimary winding of the transformer to utilize a saturable characteristicof the core of the transformer, wherein the control circuit turns offthe second switch when a current of the second switch increases.
 12. Theswitching power supply according to claim 6, further comprising: asaturable reactor connected to both terminals of the primary winding ofthe transformer to utilize a saturable characteristic of the core of thetransformer, wherein the control circuit turns off the second switchwhen a current of the second switch increases.